If a person is using an HDB91x (91xg clone) antenna for UHF-only reception, is there EVER a need for that antenna's output to be run through an FM trap filter? Does that possibility change if it's filtered through a standard UHF/VHF UVSJ combiner, with the vhf side capped? Or if the UHF antenna is pre-amplified?

Or is an FM trap filter only ever appropriate to be installed on a VHF antenna?

is there EVER a need for that antenna's output to be run through an FM trap filter?

Yes. That long boom picks up a LOT of FM, especially when situated at or near a right angle to the boom's aiming direction.

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Does that possibility change if it's filtered through a standard UHF/VHF UVSJ combiner, with the vhf side capped?

Yes, the UVSJ would be expected to attenuate everything, including FM, below the UHF port's cutoff frequency by a minimum of 20 dB. It would take an extraordinary level of FM signal to require a second stage of filtering.

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Or if the UHF antenna is pre-amplified?

I've learned the hard way that it's usually better to keep FM out of your pre-amp unless you have a very robust amp that won't overload or produce spurious signals.

A spectrum analysis would be needed to pin down specifics, so you're usually left with following "best practices" if you want to side step those issues.

I have to reconfig my antennas, and I found out the VHF side doesn't need to be amplified with the new mast height. It doesn't seem to hurt it, but it doesn't help it either. However, the UHF side does need to be amped for sure, considering my tree situation.

What I have at present, is each antenna (VHF and UHF) goes into it's respective side of a Holland UVSJ, then into an FM trap, then into a single KT-200 preamp, then down to the living room.

Here's what I propose for the new change of adding a second HDB91x just for channel 20, (3 antennas total):

'UHF All-channel' HDB91x into uhf side of a Holland UVSJ (vhf capped), then into 1st KT-200 preamp, then into the "All-channel' side of a Jointenna.

'UHF (pointed at WUWB-20)' HDB91x into uhf side of a second Holland UVSJ (vhf capped), then into my Winegard CPF-20 filter, then into a 2nd KT-200, then into the 'single channel' side of the channel 20 Jointenna.

The output of the Jointenna, will then go into the UHF side of a Radio Shack UVSJ that has power-pass on the UHF side. (this is the 3rd UVSJ, but joins all antennas for final drop to living room)

The Y10-7-13 VHF antenna, will go into an MCM Electronics FM trap, then into the VHF side of the Radio Shack UVSJ.

Then it all goes down to the power inserter, and then a splitter for my tv set and Tivo Roamio.

As a reminder, here's my original thread that solved my VHF issue permanently:

Does the above sound like it has a reasonable chance to work? I would prefer to NOT have to use two KT-200's, but I don't really see anyway around it since each HDB91x antenna's will now have to be on separate leads.